Commentators the world over refer to it, as though it were a well-established fact that US Jews wield far more influence than their numbers (2&#37; of the population) would suggest.

But this presumed influence is also a delicate issue in the US, and is rarely analysed.

How does the lobby work? Is its power truly legendary, or just a legend?

Two US academics, John Mearsheimer of the University of Chicago and Stephen Walt of Harvard, have set out to answer those questions, and triggered a firestorm of controversy as a result.

Their book The Israel Lobby and US Foreign Policy, which builds on a 2006 article in the London Review of Books, says the reasons for US support for Israel need to be explained.

America spends $3bn a year in largely military assistance - one-sixth of its direct aid budget - to help a prosperous, nuclear-armed country, and strongly backs Israel in negotiations on Middle East peace.

But according to Mearsheimer and Walt, the US gets remarkably little in return.

They reject the argument that Israel is a key ally in America's "war on terror".

On the contrary, they contend, US patronage of Israel fuels militant anger - as well as fostering resentment in Arab countries that control vital oil supplies.

One-sided

The authors also reject the common view of Israel as a democratic outpost that needs protection from deadly enemies.

It is indeed a vibrant democracy, they say, but also a regional giant ready to use its considerable firepower against civilians.

If both these arguments are weak, they say, the real reason behind US support for Israel is domestic - the activities of the American Israel Public Affairs Committee (Aipac), the Anti-Defamation League (ADL), and like-minded groups and think tanks.

Mearsheimer and Walt do not talk of a "Jewish lobby", as these groups do not speak for all US Jews and include many non-Jews, but of an "Israel lobby", whose main aim, they say, is to convince America that its interests are aligned with those of the Israeli state.

The book analyses the lobby's sources of influence - notably its financial muscle and the reluctance of critics to speak out.

Pro-Israeli contributions to US campaigns dwarf those of Arab-Americans or Muslim groups.

Like other interest groups, the Israel lobby also influences debate by rounding on politicians and commentators who take positions it does not like - but it does it particularly effectively, according to Mearsheimer and Walt.

Those who might think of questioning US support for Israel know they are in for a fight, making it more trouble than it is worth.

The resulting lack of discussion, the book says, has skewed US policies across the Middle East.

Most controversially, it argues that the lobby played an important role in the Iraq war.

No conspiracy

Perhaps not surprisingly, Mearsheimer and Walt have unleashed a torrent of criticism - though not from Aipac, which has made no comment.

"Their conclusions are classic anti-Semitic canards - such as control of foreign policy against the interest of the US, the Jews controlling the media and getting America into war," ADL director Abraham Foxman told the BBC News website.

After reading the original article, Mr Foxman wrote a book-length rebuttal entitled The Deadliest Lies: The Israel Lobby and The Myth of Jewish Control.

Many attacks have been highly personal.

In a fierce critique of their scholarship, Israeli historian Benny Morris wrote in the New Republic of the original article: "Were 'The Israel Lobby and US Foreign Policy' an actual person, I would have to say that he did not have a single honest bone in his body."

Virtually all reviews of the book in the mainstream US press have been negative.
"They have often misrepresented our arguments badly or tried to smear us by either saying or hinting that we are anti-Semitic," Mr Walt told the BBC News website.

He and Mr Mearsheimer deny recycling old fantasies of Jewish conspiracies. Their book repeatedly states that pro-Israeli lobbying is not secretive, but conforms to the open rules of America's democratic system.

The authors regard their excoriation in the US press as a sign of the lobby's effectiveness, and point out that reviews abroad have been much more favourable.

"This in some way confirms our basic argument that it's much easier to talk about this subject outside the United States than we do inside the US," he says.

Cause and effect

However, some of Mearsheimer and Walt's US critics have been less vitriolic and harder to dismiss as angry polemic.

Robert Lieberman, a Columbia University political scientist, argues that they overstate the lobby's financial power.

"I have written articles in Haaretz that no American newspapers would touch"
Tony Judt

Mearsheimer and Walt cite cases of members of Congress losing their seats after running afoul of pro-Israeli groups which then bankrolled their opponents.

But Mr Lieberman says the contributions involved are unlikely to make a difference and the book fails to establish a clear link between lobby money and victory.

Senate Minority leader Tom Daschle lost his seat in 2004 despite the fact that he got more pro-Israel funding than any candidate that year.

"For any anecdote they come up with, you can come up with an anecdote that demonstrates the opposite," Mr Lieberman says.

Perhaps the most contentious argument in the book is the direct causal link it tries to establish between lobby activity and US Middle East policies.

But political preferences can be influenced by any number of factors, such as popular pressure, party politics or heartfelt conviction.

Although Mearsheimer and Walt do their best to discard those alternative explanations for the US pro-Israeli stance, many are unconvinced.

"Is this the manipulation of a tiny group, or is this politicians not wanting to take a stand that is unpopular with the broader public?" Walter Russell Mead, of the Council on Foreign Relations, told the BBC News website.

Mr Mead - who wrote a lengthy critique of the book in the journal Foreign Affairs - also says Mearsheimer and Walt give too vague a definition of the lobby to make any credible conclusion about its impact.

Opening up

The fact that the book invites criticism, however, is also a strength. Its scholarly, dispassionate tone is meant to encourage a debate.

"Reasonable people can disagree and one of the reasons we want to have a discussion is to get issues out in the open so people can talk about them," Mr Walt says.

Tony Judt - a prominent historian and critic of Israel - does not accept every point made by Mearsheimer and Walt, but he credits them with lifting a taboo.

The main effect of the lobby, he says, has been self-censorship. "There are people out there who are anti-Semitic obviously, and you don't want to find yourself in their company, so you end up saying nothing," he says.

Mr Judt himself is not afraid to speak out, but he has to tread more carefully when he criticises Israeli policies in the US than he does in Israel itself.

"I have written articles in Haaretz that no American newspapers would touch," he says.

In this context, he adds, Mearsheimer and Walt's book is an "enormous act of intellectual courage".

"They gained nothing from it, but the community has really gained something because with each little step like that, the conversation opens up a bit more."

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